The field of the invention is that of forming integrated circuits with a buried insulator, e.g. oxide, that is formed in selected areas.
The advantages of circuits with buried oxide are well known, as are the problems associated with having the transistor body isolated from the substrate and with the extra cost associated with the long time required to perform the implant.
Extensive work has gone into various schemes for forming body contacts to alleviate the problems, but they all have problems, usually excessive consumption of silicon area.
It has been suggested to implant the oxygen ions in a patterned fashion and subject the wafer to high temperature annealing, but that still has the extra cost associated with the high dose implant and isolation of defects and oxygen precipitates from the device area.
The invention relates to an integrated circuit having buried insulator formed only under the sources and drains of transistors.
A feature of the invention is the implantation of a dopant species at a dose two orders of magnitude less than is required for oxygen implantation.
Another feature of the invention is the selective etching of the implanted areas after or during the shallow trench etch.
Yet another feature of the invention is the deposition of oxide in the buried etched cavities.